Bird that's small and pale (4)
I believe the answer is:
swan
'bird' is the definition.
(I know that swan is a type of aquatic bird)
'small and pale' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'pale' becomes 'wan' (wan can mean pale or grey).
's'+'wan'='SWAN'
'that's' is the link.
(Other definitions for swan that I've seen before include "Mute, whooper or Bewick's bird" , "Large, long-necked bird" , "Whooper, perhaps" , "Long-necked water bird" , "The Ugly Duckling's outcome" .)